The pump light source for a diode-pumped high-performance solid state laser may be a diode laser arrangement that is actively cooled by microchannel heat sinks. Diode laser arrangements may include vertical diode laser stacks (vertical stacks) of laser diode bars. With a sufficiently large stack height (e.g., approximately fifty laser diode bars), these vertical stacks have approximately identical beam parameter products in a horizontal, or “slow-axis,” direction and in a vertical, or “fast-axis,” direction. A large number of emitting areas are uniformly distributed in each laser bar in the slow-axis (“SA”) direction. The width of a bar in the SA direction is typically 10 mm. The full-angle divergence of the exiting laser radiation in the SA direction is typically approximately 6-10°. Emission in the fast-axis (“FA”) direction from the individual laser bars is achieved through an aperture of a height of approximately 1 μm and has a full-angle divergence of approximately 40-70°. The laser beams emitted from each laser bar are usually collimated in the FA direction using cylindrical microlenses. Usually, each laser bar has an individual microlens. The collimated beams typically extend 0.8-1 mm in the FA direction. The full-angle divergence of the collimated beams after passing through the microlens is approximately 0.5-2° in the FA direction. The quality of the collimating lens, the accuracy of the lens' alignment, and the straightness of the laser bar determine the divergence angle.
Vertical stacking of the laser bars facilitates generation of a laser beam having a uniform beam divergence, which facilitates coupling of the combined laser beam into an inlet opening of an almost rotationally symmetric light mixer. The pump radiation from the vertical stack of laser bars is homogenized within the light mixer, for example, through multiple reflections. The radiation emitted from the light mixer is suitable for imaging a round spot on a laser crystal to pump the laser crystal. However, the limited life expectancy of the actively cooled vertical stacks, approximately 1000 hours, is disadvantageous.
German patent publication DE 100 61 265 A1 discloses quasi-passively cooled diode laser arrangements with horizontal laser diode stacks as the pump light source for high-performance solid state lasers. The stacks are two-dimensional, passive, horizontal stacks. The individual laser bars of these diode laser arrangements are not disposed on top of each other in the FA direction. Rather, the individual laser bars are next to each other in the SA direction and slightly offset in steps in the FA direction. In general, horizontal stacks are more reliable than vertical stacks because the electrical elements are separate from the cooling elements. Additionally, horizontal stacks typically have larger cooling structures and a greatly reduced number of sealing elements.
German patent publication DE 102 29 711 A1 discloses a cooling structure for horizontal laser diode stacks. The individual laser bars of these stacks are initially mounted on passive copper heat sinks. A plurality, typically four to twelve, of the heat sinks are subsequently soldered onto a common active cooler. The active cooler includes two or more thermally conducting ceramic plates. Between the plates, there are several copper layers that include milled or etched cooling channels. The layers are connected to each other as well as to the ceramic plates through the so called “direct copper bond” method. The cooling channels are wider and the cooling surface is larger compared to conventionally cooled vertical stacks, which can reduce the flow velocity of the cooling water and problems due to abrasion of the cooling channels.
The beam produced by laser diode bars disposed next to each other in the SA direction is can be disadvantageous. In contrast to a vertical stack, the asymmetry of the beam quality from the horizontal stack is further increased by the adjacent bars and the resulting widening of the overall cross-section of the emitted radiation along the SA direction. Moreover, the comparably large size of the horizontal stacks in the FA direction requires FA collimator lenses with an unusually large focal length for optically aligned stacking in the FA direction. The small divergence of the beams collimated in this manner requires very high precision of the optical components in the optical path. Additionally, the overall height of the number of stacked horizontal stacks required for a symmetrical beam quality is too large for practical applications. For this reason, beam shaping optics are required to obtain symmetry of the output radiation of horizontal stacks, which rearranges beams of a horizontal stack extending next to each other in the SA direction into beams extending on top of each other in the FA direction.
European patent publication EP 1 059 713 A2 discloses a beam shaping unit for generating a laser beam with symmetric brightness from several parallel laser beams which are emitted from the individual diodes of a laser diode bar and are laterally offset in the SA direction. In one embodiment, the beam shaping optics include a first transmissive optical element for lateral displacement of the individual laser beams in the FA direction. A second transmissive optical element deflects the laser beams in the plane formed by the SA direction and the beam directions of the individual laser beams in such a manner that the laser beams converge towards a common axis of intersection in the FA direction. The laser beams enter along the axis of intersection from different directions, and a fanned mirror is provided at the common axis of intersection. The fanned mirror includes a number of individual mirrors corresponding to the number of laser beams; the individual mirrors are rotated relative to each other and stacked on top of each other in the FA direction. The fanned mirror deflects the laser beams into a common emission direction.